A fuel cell is a device including a MEA (Membrane-Electrode Assembly; electrolyte membrane-electrode assembly) and configured to generate electric power and heat in such a manner that: both main surfaces of the MEA are respectively exposed to a hydrogen-containing anode gas and an oxygen-containing cathode gas, such as air, to cause the anode gas and the cathode gas to electrochemically react with each other.
Generally, the fuel cell includes as a main body a stack configured by stacking unit cells. Each of the cells is configured such that the MEA is sandwiched between a pair of flat plate-shaped separators, specifically an anode separator and a cathode separator. The MEA is configured to include a polymer electrolyte membrane and a pair of electrodes respectively stacked on both surfaces of the polymer electrolyte membrane. Electrode surfaces are respectively formed on both main surfaces of the MEA. Each of the separators is made of an electrically conductive material, such as resin containing electrically conductive carbon or metal, and contacts the electrode surface of the MEA to serve as a part of an electric circuit.
Here, an electrochemical reaction in the cell is an exothermic reaction. Therefore, during an electric power generating operation of the fuel cell, the cells need to be cooled such that the temperature of an inner surface of each cell becomes a catalytic activity temperature, and the temperature of the inside of the fuel cell needs to be appropriately controlled. To be specific, if the cell is not adequately cooled, the temperature of the MEA increases, and moisture evaporates from the polymer electrolyte membrane. As a result, the durability of the cell stack decreases by the acceleration of deterioration of the polymer electrolyte membrane, and the electric output of the cell decreases by the increase in the electrical resistance of the polymer electrolyte membrane. In contrast, if the cell is cooled beyond necessity, the condensation of the moisture in a reactant gas flowing through a gas channel occurs, and the amount of water in liquid form in the reactant gas increases. The water in liquid form adheres as liquid droplets by surface tension to the gas channel of the separator plate. If the amount of liquid droplets is too large, the water adhering to the gas channel closes the gas channel and blocks the flow of the gas, and this causes flooding. As a result, the reactive area of the electrode decreases, and the performance of the fuel cell decreases, for example, the electric output becomes unstable.
Because of the above reasons, a cooling medium manifold through which a cooling medium flows is generally formed on the fuel cell stack so as to extend in a stack direction of the cells. In addition, cooling medium channels are formed among the stacked cells of the fuel cell stack so as to communicate with the cooling medium manifold. Moreover, a material with good heat conductivity is used as a material of the separator.
As the cooling medium flowing through the cooling medium manifold and the cooling medium channels, water is generally used. In a case where water, such as ion exchanged water, with low electric conductivity is used as the cooling medium, short-circuit occurs between positive and negative members in the fuel cell stack, specifically between the electrically conductive separators, via the cooling water filled in the cooling medium manifold. This is because such water is electrically conductive. As a result, a short-circuit current flows through the separator, and this causes the corrosion of a wall surface forming a cooling medium manifold hole formed on a positive separator.
Here, a fuel cell is known, in which to solve the problem of the corrosion of the separator, a projecting portion is formed on an inner side of a cross section, relative to the stack direction of the fuel cell, of each of a cooling liquid supply manifold and a cooling liquid discharge manifold (see PTL 1, for example).